This invention relates to electronic means for controlling mechanical vibration (typically due to resonances) in disk drive servo systems or similar servo systems. In particular, the invention is described with reference to a servo system used to drive a head assembly radially with respect to rotatable disk storage media, and comprises a method whereby mechanical vibration excited by forces due to a particular severe combination of seek commands can be prevented. Advantageously, prior knowledge of what particular seek commands cause the vibration is not required.
It is presently common practice in the data processing industry to store large quantities of data in digital form upon rotating storage media. Such disk storage media has heretofore been realized with magnetic media, although significant strides are presently being taken to utilize an optical storage disk for this same purpose.
Regardless of the type of disk or media that is employed, i.e., magnetic or optical, some type of read/write head, typically one for each side of each disk, must be moved in and out radially with respect to the disk in response to machine commands. The heads are part of a head assembly and the head assembly is moved as an integral unit until the particular read or write head is positioned at the distance from the center of the disk where the particular record sought to be read or written is located.
It is, of course, very desirable to provide storage media with as much data storage capacity as possible. To this end, it is desirable to locate data tracks on the disk storage media as close to one another as possible. This means that the drive means used to move the head assembly radially with respect to the disk must be accurately controlled. The usual drive choice is a servo system in which a motor, typically a linear motor (although other types of motors or actuators could be employed), is used to drive the head assembly back and forth with respect to the disk, and in which position signals relative to the actual position of the head assembly are generated by sensing, with an appropriate transducer, permamently encoded position information on the disk itself, thereby providing an accurate position sensing means. The information gained from the transducer as to the actual position of the head assembly relative to the disk is then compared to a desired location on the disk where desired data is to be found or written. This combined information is then used to generate a position command which is used to control the action of the linear motor such that the head assembly is radialy moved with respect to the disk until the desired position is reached. Such "feedback" servo systems are well known in the art.
The high performance requirements of modern data storage media pose considerable difficulty, however, to conventional "feedback" servo systems. For example, it is desirable to treat disk storage media as "Random Access Memory"--that is, memory which may be non-sequentially accessed--which requires that the access time of the disk storage media be as short as possible. If this facility is to be economically feasible with today's high speed computers, the servo system must be increasingly accurate and fast. This in turn leads to difficulties with mechanical vibration that develops in the servo system, including the head assembly.
With magnetic disk, in order that the data may be packed densely thereon, it is necessary that the magnetic heads "fly" on the magnetic surface on an air bearing having a thickness on the order of several microinches. Clearly, only a minor vibration is required to destroy such a delicate air bearing, and it is therefore essential that excitation of mechanical resonances be avoided.
With respect to optical disk, the data density is at least an order of magnitude greater than that realized on magnetic disk. The optical heads must have the capability of finding an area or band that is only a few microns wide. Again, only a minor resonance is required to destroy the fine resolution that is required.
A further consideration which must be addressed by a designer of a servo system is the accurate position of the head assembly relative to the disk so that data can be reproducablly read or written onto specific areas of each individual disk. This consideration requires that the position-sensing transducer employed be extremely accurate and repeatable. Again, mechanical vibration can cause such transducers to produce inaccurate data.
Numerous systems have been proposed in the prior art for generating accurate position sensing information. For example, see e.g., U.S. Pat. No. 3,820,712 to Oswald for "Electronic Tachometer"; and U.S. Pat. No. 4,321,517 to Touchton et. al. for "Resonance Suppression Method." The Oswald reference, however, does not address the problem of mechanical resonance or vibration. The Touchton reference does address the suppression of resonances that occur during a seek operation ("seek" being hereinafter defined as the process of moving the head assembly from one location or band on the disk to another location or band), but does not address the problem of mechanical vibration or resonances that result from a particular combination of seek commands.
Because the disk is operated as a "Random Access Memory," the seek commands are likewise random. However, because the slidable head assembly represents a mechanical system having a resonant frequency, it is possible that a particular sequence of seek commands will excite a tuned condition or mechanical resonance of the mechanical system. Such excitation may cause the head assembly to undergo large vibrations which can either cause errors or limit the track density. Because such seek-excited resonances are really not detectable until after the fact i.e., after the seek operation has been completed, the only way to deal with such resonances is to allow them to die out before initiating the next seek operation. However, to add a time delay after each seek command that is sufficient for a seek-excited resonance to dampen out seriously degrades the overall seek performance (speed) of the machine. Thus, there is a need in the art for a method of predicting or estimating when such seek-excited resonances will occur and for breaking up the seek pattern only in those relatively few instances where seek-excited resonances are likely to occur.